Cold work tool steel

ABSTRACT

Cold work tool steel of the present invention comprises 0.4≦K value≦2.6 (K value Cr(wt %)−6.8C (wt %)), 15.5≦L value≦21.0 (L value=Cr(wt %)+15.5C (wt %), 0.60 wt %≦Si≦2.0 wt %, 0.10 wt %≦Mn≦1.0 wt %, 0.03 wt %&lt;S≦0.2 wt %, 1.25 wt %&lt;Mo+0.5 W&lt;3.0 wt %, 0.05 wt %≦V≦1.0 wt %, and the balance Fe and inevitable impurities, in which the highest hardness obtained by tempering at 450° C. or higher after quenching is HRC 61 or more.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cold work tool steel and, more specifically, to cold work tool steel suitable for a variety of cold dies such as a cold forging punch and die, a forming die for high tensile strength steel plate, a bending die, a cold forging die, a swaging die, a thread rolling die, a punch member, a slitter knife, a lead frame blanking die, a gauge, a deep drawing punch, a bending die punch, a shear blade, a bending die for stainless steel, a drawing die, a plastic working tool for cold press, a gear punch, a cam part, a blanking press die, a progressive blanking die, a seal plate for sediment feeder, a screw member, a rotary plate for concrete spraying machine, an IC sealing die, and a precision press die which requires high dimensional precision, and the above-mentioned cold dies used after being subjected to surface treatment such as CVD treatment, PVD treatment or TD treatment.

2. Description of the Related Art

Cold work tool steel represented by JIS steel SKD 11 is enhanced in wear resistance by dispersing a large amount of high-hardness carbides by crystallization or precipitation, and used for various purposes (e.g., cold forging punch and die, cold forging die, etc.) that require wear resistance or galling resistance. However, the conventional cold work tool steel has problems such as (1) insufficient toughness; (2) weakening of die hardness with hardening of a forming condition or the like; and (3) probability of cracking in wire electric discharge machining.

Therefore, various proposals are conventionally made to solve these problems. For example, Patent Reference 1 discloses cold work tool steel comprising C: 0.75 to 1.75 wt %, Si: 0.5 to 3.0 wt %, Mn: 0.1 to 2.0 wt %, Cr: 5.0 to 11.0 wt %, Mo: 1.3 to 5.0 wt %, V: 0.1 to 5.0 wt %, and the balance Fe and impurities, which is tempered at a temperature of 450° C. or higher. It also describes that the toughness is improved by minimizing a primary eutectic carbide, and the tool life and electric discharge machinability are significantly improved by enhancing secondary hardening hardness by tempering at 450° C. or higher while adjusting each component.

Patent Reference 2 discloses cold work tool steel having a predetermined composition with an agglomeration size of carbide agglomeration parts of 100 μm or less. It also describes that cracking of carbides and propagation thereof are inhibited by setting the agglomeration size to 100 μm or less to improve the tool life.

Patent Reference 3 discloses high-hardness cold work tool steel having a predetermined composition wherein α value (=0.706+0.541C−0.063Cr+0.033 Mo−0.232V) is 0.7 to 1.0, and 0 value (=Mo equivalent+1.9V equivalent) is 3.0 to 6.0. It also describes that formation of coarse carbides and agglomerated carbides is inhibited by setting a value and value to these ranges, and the adhesion with a hard surface layer is thereby improved.

Patent Reference 4 discloses cold work tool steel having a predetermined composition in which 5 to 35 vol. % of retained austenite is finely dispersed in an average grain diameter of 0.01 to 2 μm. It also describes that the fatigue resistance is improved by finely dispersing a predetermined amount of retained austenite.

Further, Patent Reference 5 discloses steel having a predetermined composition in which a free-cutting element is added while reducing C content to improve the machinability. It also describes that the residual stress can be removed by high-temperature tempering to prevent the cracking by electric discharge machining.

[Patent Reference 1]

Japanese Patent Application Laid-Open No.S 59-179762

[Patent Reference 2]

Japanese Patent Application Laid-Open No. 2002-12952

[Patent Reference 3]

Japanese Patent Application Laid-Open No. 2000-073142

[Patent Reference 4]

Japanese Patent Application Laid-Open No. 2004-035920

[Patent Reference 5]

Japanese Patent Application Laid-Open No. 2000-355737

Experimental dies and small-lot-number dies require, particularly, excellent workability much more than die life. Namely, some kinds of use need cold work tool steel capable of ensuring HRC 60 or more that is the same hardness as general dies while keeping easiness of working in any of working methods including cutting, electric discharge machining, and wire electric discharge machining.

However, the conventional cold work tool steel represented by SKD 11 has the problem of poor machinability even in an annealed state because crystallized carbides are dispersed in a large amount to ensure predetermined wear resistance. Further, breaking of wire is often caused by the crystallized carbides at the time of wire electric discharge machining.

A material in which the amount of crystallized carbides is reduced to improve the workability is also known. However, in such a conventional material, it is difficult to ensure HRC 60 or more at the time of high-temperature tempering. On the other hand, when this material is tempered at a low temperature to obtain high hardness, the residual stress generated in the material at the time of quenching cannot be removed. Therefore, when electric discharge machining or wire electric discharge machining is performed to the resulting material, the residual stress may lose the balance to cause the crack or breakage of the material.

In the cold work tool steel disclosed in Patent Reference 1, since the crystallized carbides are reduced more than in SKD11, and component adjustment is performed, a hardness of HRC 60 or more can be ensured, and the workability is improved to some degree. However, even in the cold work tool steel disclosed in Patent Reference 1, the improvement in workability is insufficient. Patent References 2 to 5 disclose no concrete means for improving the workability while keeping high hardness.

SUMMARY OF THE INVENTION

The present invention thus has an object to provide cold work tool steel excellent in workability with a hardness of HRC 60 or more after tempering.

The cold work tool steel according to the present invention comprises:0.4≦K value≦2.6 (K value=Cr(wt %)−6.8 C (wt %)); 15.5≦L value≦21.0 (L value=Cr(wt %)+15.5 C (wt %)); 0.60 wt %<Si≦2.0 wt %; 0.10 wt %≦Mn≦1.0 wt %; 0.03 wt %<S≦0.2 wt %; 1.25 wt %<Mo+0.5 W≦3.0 wt %; 0.05 wt %≦V≦1.0 wt %, and the balance Fe and inevitable impurities; in which the highest hardness obtained by tempering at 450° C. or higher after quenching is HRC 61 or more.

According to the cold work tool steel of the present invention, since the K value is set to a predetermined range, a maximum hardness after high-temperature tempering of HRC 61 or more can be obtained. Since the L value is set to a predetermined range, machinability can be improved, and the breaking of wire in wire electric discharge machining can be inhibited. Further, since 0.6 wt % or more of Si is added in addition to S, machinability equal to or more than in a conventional free-cutting steel can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the relation between L value and quantity of crystallized carbides; and

FIG. 2 is a view showing the relation between Si amount and machinability.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention will be described in detail. The cold work tool steel of the present invention comprises the following elements and the balance substantially Fe and inevitable impurities. The kinds of elements to be added, and the component ranges and reasons of restriction thereof are as follows. 0.4≦K value≦2.6 (K value=Cr (wt %)−6.8 C (wt %))   (1)

The K value shows the amount of residual Cr in the matrix at a proper quenching temperature. When the K value is less than 0.4 or exceeds 2.6, the amount of carbides formed which contribute to secondary hardening in tempering is reduced to make it difficult to obtain a hardness of HRC 61 or more by high-temperature tempering of 450° C. or higher. The K value is further preferably 0.45 or more and 2.5 or less and, more preferably, 0.6 or more and 2.4 or less. 15.5≦L value≦21.0 (L value=Cr (wt %)+15.5 C (wt %))   (2)

The L value shows the amount of crystallized carbides contained in the material. The larger the L value is, the more the amount of the crystallized carbides is increased. When the L value is less than 15.5, a necessary hardness cannot be obtained since not only the crystallized carbides are hardly formed, but also the matrix component at a proper quenching temperature is changed. When the L value exceeds 21.0, the amount of the crystallized carbides excessively increases to deteriorate the machinability, electric discharge machinability, and toughness. However, it would be better to leave the crystallized carbides to some degree since perfect elimination of the crystallized carbides causes coarsening of grains or inclusion of various grain sizes at the time of quenching. The L value is further preferably 15.8 or more and 20.8 or less and, more preferably, 16.0 or more and 20.5 or less.

The crystallized carbides mean carbides exceeding about 10 μm in terms of circle-corresponding diameter, and mainly represented by M₇C₃ (M is Cr, Mo, etc.). The L value of 15.5 to 21.0 corresponds to 0.20 to 4.0 wt % in terms of the weight ratio of the crystallized carbides. 0.60 wt %<Si≦2.0 wt %   (3)

Since Si is added as a deoxidizing element, it is generally included in steel. In the present invention, Si is actively added in order to facilitate cutting. The improvement in machinability by addition of Si can be obtained not only in a low-hardness (about HRB 95) state after annealing but also in a high-hardness (HRC 61 or more) state after quenching and tempering. The addition of Si contributes also to the improvement in high-temperature tempering hardness.

To obtain such an effect, addition of Si in an amount exceeding 0.6 wt % is needed. Even if Si is excessively added, the effect is saturated. Accordingly, the Si amount is preferably set to 2.0 wt % or less. The amount of Si is further preferably 0.65 wt % or more and 1.8 wt % or less and, more preferably, 0.70 wt % or more and 1.5 wt % or less. 0.10 wt %≦Mn≦1.0 wt %   (4)

Mn has the effect of enhancing hardenability to improve the hardness and strength. It reacts with S which is a free-cutting element, thereby forming an inclusion to effectively improve the cutting property. To obtain such an effect, addition of Mn in an amount of 0.10 wt % or more is needed. When Mn is excessively added, hot workability is deteriorated. Accordingly, the Mn amount is preferably 1.0 wt % or less. 0.03 wt %≦S≦0.20 wt %   (5)

S is a free-cutting element, which is bonded to Mn to form an inclusion to improve machinability. The improvement in machinability by the addition of S can be obtained not only in a low-hardness (about HRB 95) state after annealing but also in a high-hardness (HRC 61 or more) state after quenching and tempering.

To obtain such an effect, addition of S in an amount of 0.03 wt % or more is needed. When the S amount is excessively large, the Charpy impact value in the material section is largely deteriorated, and hot workability is also deteriorated. Accordingly, the S amount is preferably 0.20 wt % or less. 1.25 wt %<Mo+0.5 W<3.0 wt %   (6)

Mo and W form carbides to increase the secondary hardening quantity in tempering at 450° C. or higher. Although Mo and W bring the same effect, W needs a double amount to have the effect equal to Mo. Therefore, the amount of Mo and W is regulated by the Mo equivalent described by Mo+0.5 W.

To obtain a hardness of HRC 61 or more after quenching and tempering, the Mo equivalent must be larger than 1.25 wt %. However, when the Mo equivalent is too large, hot workability, toughness and machinability are deteriorated. Accordingly, the Mo equivalent is preferably less than 3.0 wt %. 0.05 wt %≦V≦1.0 wt %   (7)

V forms stable carbides to effectively prevent the coarsening of grains. It also contributes to improvement in wear resistance or hardness by the formation of the carbides. To obtain these effect, addition of V in an amount of 0.05 wt % or more is needed. When the amount of V is too large, deterioration of machinability and hot workability is caused by an increase in carbide quantity. Accordingly, the V amount is preferably 1.0 wt % or less.

The cold work tool steel of the present invention may further include one or more elements as described below in addition to the above-mentioned elements. The component range and reason of restriction of each element are as follows. 0.005 wt %≦Se≦0.10 wt %   (8) 0.005 wt %≦Te≦0.10 wt %   (9) 0.0002 wt %≦Ca≦0.010 wt %   (10) 0.005 wt %≦Pb≦0.10 wt %   (11) 0.005 wt %≦Bi≦0.10 wt %   (12)

Se, Te, Ca, Pb and/or Bi may be added for the purpose of improving machinability. The addition of these elements never inhibits the improvement in machinability by the addition of Si.

Se and Te can be used as alternate elements of S in Mn-sulfides. Ca improves the machinability by forming an oxide or being dissolved in Mn-sulfide to form a protective film on the surface of a cutting tool at the time of machining. Further, Pb and Bi, which are low melting point materials, are melted by the heat generated by machining to produce a lubricating effect between the cutting tool and chips, improving the machinability.

To obtain the effects, the addition of the above-mentioned lower limits or more is needed. Since excessively large addition amounts of these elements cause deterioration of mechanical characteristics, the addition amounts are preferably set to the above-mentioned upper limits or less. 0.01 wt %≦Cu≦2.0 wt %   (13) 0.01 wt %≦Ni≦2.0 wt %   (14) 0.20 wt %≦Co≦1.0 wt %   (15) 0.0003 wt %≦B≦0.010 wt %   (16)

Cu, Ni, Co and B are dissolved in the matrix to effectively improve hardenability. Ni also has the effect of improving the toughness by reducing the impact transition temperature and preventing the deterioration of weldability by the improvement in toughness. In a cold die, the die temperature is often locally raised by the working-heating depending on high tensile strength steel or the working condition. Co has the effect of improving the high-temperature strength to prevent the permanent set of the die by such a temperature rise. To obtain these effects, the addition amounts of these elements are preferably set to the above-mentioned lower limit values or more. Since extremely large addition amounts of the elements cause deterioration of mechanical characteristics, the addition amounts are preferably set to the above-mentioned upper limits or less. 0.001 wt %≦P≦0.030 wt %   (17) 0.0050 wt %≦N≦0.050 wt %   (18) 0.001 wt %≦Al≦0.10 wt %   (19) 0.0002 wt %≦O≦0.010 wt %   (20)

P, N and O are inevitably included in steel. P segregates in a grain boundary, O forms oxides, and N forms nitrides. Al reacts with O or N in steel to form oxides or nitrides. These elements can improve the toughness by reducing their addition amounts. To obtain such an effect, the addition amounts of these elements are desirably set to the above-mentioned upper limit values or less. Desirably, P is 0.020 wt % or less, N is 0.030 wt % or less, Al is 0.050 wt % or less, and O is 0.050 wt % or less.

Oxides or nitrides of Al contribute to the prevention of coarsening of grains. Therefore, when these elements are excessively reduced, the grains become inversely coarse to deteriorate the toughness. To reduce the amounts of these elements more than necessary leads to an increase in manufacturing cost. Further, when these elements reach certain fixed values or less, the effect of improving the toughness is also saturated. Accordingly, the amounts of these elements are preferably set to the above-mentioned lower limit values or more. 0.010 wt %≦Nb≦0.10 wt %   (21) 0.005 wt %≦Ta≦0.10 wt %   (22) 0.005 wt %≦Ti≦0.10 wt %   (23) 0.005 wt %≦Zr≦0.10 wt %   (24) 0.005 wt %≦Mg≦0.10 wt %   (25) 0.005 wt %≦REM≦0.10 wt %   (26)

Nb, Ta, Ti, Zr, Mg and REM each effectively improve the toughness. Among them, Nb, Ta, Ti and Zr form fine carbonitrides and effectively improve the toughness by fining the grains. Mg and REM effectively improve the toughness by reducing the oxygen amount in the matrix.

To obtain such effects, the addition amounts of these elements are preferably set to the above-mentioned lower limit values or more. When the addition amounts are excessively large, deterioration of toughness and weldability is caused. Accordingly, the addition amounts of these elements are preferably set to the above-mentioned upper limit values or less.

The cold work tool steel of the present invention is obtained by quenching and tempering a material having a composition as described above. When the tempering temperature is low, the release of the residual stress introduced at the time of quenching becomes insufficient to deteriorate electric discharge machinability. Accordingly, the tempering temperature is preferably set to 450° C. or higher. Since the alloy components are optimized in the cold work tool steel of the present invention, high hardness (concretely, a highest hardness of HRC 61 or more) can be obtained even if tempering is carried out at a high temperature of 450° C. or higher.

The grain diameter of the prior austenite has an influence on toughness. To obtain cold work tool steel having high toughness, a smaller grain diameter of the prior austenite is preferred. However, when the grain diameter is too small, the effect is minimized, and an increase in cost is rather caused. Accordingly, the grain diameter of the prior austenite is preferably set to 3.0 or more and 8.0 or less in terms of grain size Gq. The “grain size Gq” means the grain size of the prior austenite after quenching, which is measured by use of a method described in JIS G0551.

The carbides included in cold work tool steel have the effect of preventing the coarsening of grain diameter at the time of quenching or the like. However, since the carbide amount is set small in the present invention, compared with conventional cold work tool steel such as SKD 11, the grain diameter becomes relatively easily coarse. Accordingly, in order to obtain high toughness with an appropriate grain size Gq, it is needed to perform the quenching treatment at a proper temperature. Concretely, the quenching temperature is preferably 950° C. or higher and 1080° C. or lower. When the quenching is performed in this temperature range, the coarsening of grain diameter can be prevented.

Since the present invention basically intends to enhance the free cutting property by addition of S, A type inclusions are desirably within a certain fixed range. The “A type inclusions” mean inclusions determined by use of an inclusion evaluation method described in JIS G0555, which mainly correspond to sulfides.

To obtain cold work tool steel excellent in machinability, dA60×400 is preferably within the range of from 0.10% to 1.50%. The “dA60×400” means the content of inclusion of the A type inclusions measured based on a method described in JIS G0555, which is the content of inclusion in 60-FOV (field of view) observation with a 400-power optical microscope. To obtain further high machinability, the ratio of A type inclusions with a maximum length of 20 μm or less is preferably 30% or more of the whole A type inclusions.

To form such A type inclusions, Mn must be added in an amount suitable to the S amount. As the Mn amount wt %, 1.7×S amount wt % or more is needed at a minimum. Since Mn is needed also to enhance the hardenability, Mn is generally added in an amount larger than the Mn amount wt % suitable to the S amount wt %.

Since B type inclusions and C type inclusions (alumina, other oxides, etc.) not only obstruct enhancement in free cutting property but also cause deterioration of the Charpy impact value, they are preferably reduced as much as possible. The “B type inclusions” and “C type inclusions” mean inclusions determined by use of the inclusion evaluation method described in JIS G0555.

To obtain cold work tool steel excellent in free cutting property and impact resistance, concretely, d(B+C) 60×400 is preferably 0.05% or less. The “d(B+C) 60×400” means the content of inclusion of the B type inclusions and C type inclusions measured based on a method described in JIS G0555, which is the content of inclusion in 60-FOV (field of view) observation with a 400-power optical microscope.

The effect of the cold work tool steel of the present invention will be then described. Since the crystallized carbides have a high hardness, the wear resistance of the cold work tool steel can be enhanced by dispersing the crystallized carbides in a large amount. However, a large amount of the crystallized carbides not only deteriorates the machinability but also causes the breaking of wire in wire electric discharge machining. The crystallized carbides are apt to be starting points of cracks because their grain sizes are generally large. On the other hand, an excessively small amount of the crystallized carbides causes reduction in hardness, coarsening of grains, and deterioration of toughness.

In the cold work tool steel of the present invention, since the amount of the crystallized carbides is relatively reduced while optimizing the L value, the machinability is improved, and a trouble such as breaking of wire in wire electric discharge machining is also reduced. Further, since the amount of coarse crystallized carbides which are starting points of cracks is minimized, and the grain becomes also fine, high toughness can be obtained.

The K value shows the amount of residual Cr in the matrix at a proper quenching temperature as described above, and necessary secondary hardening can be attained by optimizing the K value. Si is a free-cutting element and also contributes to improvement in hardness over the tempering temperature. Further, the Mo equivalent has an influence on the secondary hardening hardness.

The cold work tool steel of the present invention can ensure a hardness of HRC 61 or more which is necessary as cold work tool steel at the time of quenching and tempering, since other components such as Si amount and Mo equivalent are optimized in addition to optimization of the K value.

Further, since high-temperature tempering can be performed, the residual stress in the material generated at the time of quenching can be sufficiently released. Therefore, the crack or breakage can be prevented even at the time of electric discharge machining or wire electric discharge machining, in addition to excellent machinability.

Further, the material is apt to be welded to a cutting face in machining in a high-speed range (with high rotating speed). Therefore, the abrasion of the tool is easy to progress by repeated formation and separation of the weld part. In the cold work tool steel of the present invention, since Si is added in an amount of 0.6 wt % or more, the welding is hardly caused, and the abrasion of the tool can be suppressed. Therefore, this cold work tool steel can retain higher workability than the conventional free-cutting steel.

EXAMPLES

A steel material of 80 kg having each component composition shown in Table 1 (Examples 1 to 20 and Comparative Examples 1 to 10) was melted in a high-frequency vacuum melting furnace. A steel ingot obtained by casting it was hot forged to form a square bar of 35×55 mm. After the hot forging, spheroidizing for gradually cooling from 880° C. at a cooling rate of 7° C./hr was performed. TABLE 1 No C Si Mn P S Cu Ni Cr Mo W V Examples 1 0.75 0.91 0.43 — 0.065 — — 6.75 1.89 0.02 0.21 2 0.76 0.61 0.55 — 0.085 — — 5.73 1.25 0.04 0.23 3 0.89 0.65 0.32 — 0.093 — — 6.73 1.32 — 0.19 4 0.90 0.73 0.38 — 0.134 — — 7.03 1.33 — 0.17 5 0.83 0.83 0.68 — 0.153 — — 8.03 1.53 — 0.14 6 0.73 0.93 0.83 — 0.194 — — 7.42 1.73 0.06 0.11 7 0.62 1.03 0.92 — 0.034 — — 6.64 1.59 — 0.09 8 0.60 1.12 0.98 — 0.043 — — 6.21 1.39 — 0.07 9 0.74 1.32 0.12 — 0.053 — — 6.43 1.95 — 0.05 10 0.73 1.53 0.25 — 0.083 — — 7.39 1.83 0.11 0.12 11 0.68 1.73 0.23 — 0.075 0.09 0.21 6.33 2.21 — 0.32 12 0.83 1.83 0.21 — 0.087 0.03 0.81 6.78 2.39 0.31 0.33 13 0.79 1.91 0.53 — 0.093 0.31 0.08 7.34 2.57 — 0.36 14 0.66 1.03 0.22 — 0.063 0.03 0.23 6.48 2.85 — 0.41 15 0.75 0.95 0.34 0.007 0.068 0.11 0.09 6.11 2.93 — 0.44 16 0.79 0.94 0.41 0.004 0.068 0.21 0.39 5.98 2.71 — 0.67 17 0.81 0.83 0.53 0.001 0.073 0.12 0.11 7.84 2.13 — 0.78 18 0.83 0.89 0.44 0.015 0.041 0.33 0.04 8.10 2.31 0.53 0.83 19 0.85 1.23 0.38 0.029 0.051 0.21 0.08 7.45 2.09 — 0.92 20 0.74 0.94 0.39 0.021 0.073 0.23 0.13 6.45 1.89 1.03 0.81 Comparative 1 1.03 0.94 0.35 0.013 0.001 0.21 0.13 8.31 1.94 1.53 0.23 Examples 2 0.63 0.73 1.43 — 0.011 — 0.21 6.53 1.02 — 0.31 3 0.69 0.43 0.43 — 0.045 — — 6.94 0.34 1.35 0.21 4 0.75 0.32 0.13 — 0.073 0.03 — 6.23 0.86 1.04 0.01 5 0.71 0.21 0.23 0.032 0.063 0.09 0.31 6.01 0.95 0.09 0.02 6 1.35 0.63 0.45 0.021 0.001 — — 12.31 1.93 — 1.35 7 1.24 0.93 0.83 0.011 0.002 0.34 — 11.83 1.83 0.35 0.41 8 1.11 1.32 0.03 — 0.002 — 0.02 10.84 2.01 — 0.24 9 1.04 0.85 0.29 0.019 0.008 0.22 — 8.83 1.67 0.46 0.27 10 1.03 0.32 0.63 — 0.103 — — 7.34 1.23 0.19 0.33 No Al N O K value L value Others Examples 1 — — — 1.65 18.375 2 — — — 0.562 17.51 3 — — — 0.678 20.525 4 — — — 0.91 20.98 5 — — — 2.386 20.895 6 — — — 2.456 18.735 7 — — — 2.424 16.25 Se = 0.011 8 — — — 2.13 15.51 Ca = 0.0054 9 — — — 1.398 17.9 Pb = 0.007, Bi = 0.084 10 — — — 2.426 18.705 Te = 0.083 11 — — — 1.706 16.87 Co = 0.43 12 — — — 1.136 19.645 13 — — — 1.968 19.585 Se = 0.031, B = 0.0007 14 — — — 1.992 16.71 15 0.011 0.023 0.0034 1.01 17.735 Ca = 0.0049 16 0.031 0.011 0.0023 0.608 18.225 17 0.021 0.008 0.0053 2.332 20.395 18 0.043 0.005 0.0084 2.456 20.965 Mg = 0.084 19 0.059 0.038 0.0004 1.67 20.625 Nb = 0.09, Ta = 0.007, Zr = 0.09 20 0.085 0.045 0.0013 1.418 17.92 Ti = 0.006, REM = 0.084 Comparative 1 0.003 0.011 0.0019 1.306 24.275 Examples 2 — 0.009 — 2.246 16.295 3 0.004 0.1128 — 2.248 17.635 4 — — 0.0001 1.13 17.855 5 — 0.034 0.0083 1.182 17.015 6 0.023 — 0.031 3.13 33.235 7 — 0.021 — 3.398 31.05 8 0.169 — 0.0005 3.292 28.045 9 — — 0.0142 1.758 24.95 10 0.034 — — 0.336 23.305 Each component composition is represented by “amount wt %.” It is the same with regard to K value and L value.

Each resulting steel product was subjected to machinability test (end mill machining test), wire electric discharge machining test, hardness evaluation, Charpy impact test, grain size Gq after quenching, and evaluation of content of inclusion.

The machinability test (end mill machining test) was performed to a test piece cut from the steel product in a spheroidizing annealed state. The test conditions are as follows. Tool: Cemented carbide M 20 (φ 32 mm) Speed:  200 m/min Feed rate: 0.15 mm/rev Cutting width:  4.5 mm Cutting height:  1.2 mm Cutting oil: none Tool life: Cutting distance at arrival of side flank maximum abrasion wear of 0.3 mm Evaluation Method: Relative evaluation with the tool life of Comparative Steel No. 1 as 100

The wire electric discharge machining test was carried out according to the following procedure. Namely, a test piece of 30×50×200 mm was cut from the spheroidizing annealed steel, and the test piece was quenched and tempered under a predetermined condition. After a hole of φ4 mm was drilled in the test piece, the test piece was bored in a square form of 10×20 mm by a wire electric discharge machine. After the electric discharge machining, the test piece was allowed to stand for one day, and the number of cracks generated in the test piece was measured.

For the hardness, a sheet-like test piece of 20×20 mm was cut from the spheroidizing annealed steel, and the hardness after quenching and tempering it at a predetermined temperature was measured. As the hardness, the value (test hardness) in tempering at a specified temperature shown in Table 2 and the maximum value (highest hardness) in tempering at 100 to 600° C. were measured.

The Charpy impact test was carried out at the room temperature for a Charpy impact test piece of 10R-notch shape formed from the spheroidizing annealed steel after quenching and tempering at a predetermined temperature. An average value of three test pieces was taken as the impact value.

The grain size Gq was measured according to a method described in JIS G0551. The content of inclusion was measured for the A type inclusions and (B+C) type inclusions by a method described in JIS G0555 (Magnification of optical microscope: 400, Field number: 60).

The quenching temperature, tempering temperature and the results of various evaluation tests are shown in Table 2. The relation between the L value and the amount of crystallized carbides is shown in FIG. 1. TABLE 2 Maximum Test Wire Electric Charpy Content of Content of Quenching Tempering Hardness Hardness Discharge Impact Grain inclusions inclusions No Temp. (° C.) Temp. (° C.) (HRC) (HRC) Machinability Machinability Value Size(Gq) dA d(B + C) Ex- 1 1020 500 62.8 61.3 187 Good 33 6.7 0.23 0.002 am- 2 1015 520 63.3 62.1 175 Good 35 7.8 0.85 0.001 ples 3 1010 490 64.3 63.1 189 Good 31 5.5 0.41 0 4 1000 450 61.8 61.8 248 Good 39 5.7 1.23 0.004 5 990 470 62.2 62.1 283 Good 34 6.3 0.93 0.003 6 1030 530 61.1 61.1 344 Good 32 4.3 1.13 0 7 1020 480 63.3 62.2 164 Good 41 4.8 0.11 0.004 8 1035 490 62.8 62.2 186 Good 41 5.8 0.29 0.001 9 1045 500 63.3 61.1 222 Good 35 4.9 0.28 0.008 10 1050 510 62.2 61.2 283 Good 32 7.9 0.75 0.003 11 1020 500 63.9 61.2 297 Good 31 6.8 0.38 0 12 1025 500 64.9 61.3 324 Good 35 3.4 0.63 0 13 1015 510 64.7 62.2 340 Good 34 3.9 0.53 0.002 14 1010 520 64.1 62.4 199 Good 45 4.5 0.35 0 15 1015 480 63.1 62.1 195 Good 49 4.1 0.45 0 16 1020 500 61.1 61.1 194 Good 41 5.9 0.43 0 17 1030 460 62.3 62.2 187 Good 47 6.3 0.36 0.002 18 1030 530 63.8 61.9 156 Good 47 6.8 0.42 0.004 19 1030 480 63.3 61.8 208 Good 38 5.2 0.26 0.005 20 1025 550 62.1 62.1 200 Good 37 6.9 0.73 0.007 Com- 1 1035 500 63.3 62.2 100 Good 12 7.3 0.004 0.003 par- 2 1030 500 59.3 59.3 123 N.G. Low hardness 33 7.4 0.002 0.004 a- 3 1020 250 61.1 61.1 133 N.G. Crack 32 4.8 0.25 0.13 tive 4 1020 500 57.3 57.3 114 N.G. Low hardness 34 5.3 0.36 0 Ex- 5 1010 490 56.7 56.7 143 N.G. Low hardness 39 5.6 0.31 0.001 am- 6 1020 490 63.9 59.3 69 N.G. Low hardness 11 4.9 0.007 0.13 ples 7 1015 480 63.3 59.2 83 N.G. Breaking of wire 9 6.3 0.012 0 8 1150 510 62.7 58.4 65 N.G. Breaking of wire 12 0.5 0.019 0 9 1030 200 61.8 61.7 95 N.G. Crack 11 5.7 0.04 0.35 10 1030 180 62.1 62.1 103 N.G. Crack 9 6.3 0.75 0.003

As shown in FIG. 1, the L value has a correlation with the amount of crystallized carbides, and when the L-value exceeds 21.0, the amount of the crystallized carbides exceeds 4.0 wt %. In each of Comparative Examples 1 and 6 to 10, the L value is relatively high, and the amount of crystallized carbides is large. Therefore, the impact value is low with poor machinability. Particularly, for Comparative Examples 7 and 8, breaking of wire was caused in wire electric discharge machining. In Comparative Examples 3, 9 and 10, cracks were caused after electric discharge machining because of the tempering temperature lower than 450° C. Further, in Comparative Examples 2, 4 and 5, the highest hardness was less than HRC 61 because alloy elements such as Mn, Mo and V are out of the range of the present invention. In Comparative Example 6, sufficient secondary hardening could not be attained because of an excessively large K value, and the highest hardness was less than HRC 61.

In contrast, each of the inventive steels (Examples) 1 to 20 has a highest hardness of HRC 61 or more since the K value, the L value and each amount wt % of other alloy elements are optimized, and shows a high impact value with excellent machinability and electric discharge machinability.

The relation between the addition amount of Si and machinability is shown in FIG. 2. FIG. 2 shows that the machinability is significantly improved when the Si amount is 0.6 wt % or more. The reason is that the tool abrasion resulting from welding is inhibited by the addition of a predetermined amount of Si.

The preferred embodiments of the present invention are described above in detail. However, the present invention is never limited by the above-mentioned embodiments, and various changes maybe made in the present invention without departing from the gist of the present invention.

The cold work tool steel of the present invention can be used as various cold working dies and various cold working tools. 

1. Cold work tool steel comprising: 0.4≦K value≦2.6 (K value=Cr(wt %)−6.8 C (wt %)), 15.5≦L value≦21.0 (L value=Cr(wt %)+15.5 C (wt %)), 0.60 wt %<Si≦2.0 wt %, 0.10 wt %≦Mn≦1.0 wt %, 0.03 wt %<S≦0.2 wt %, 1.25 wt %<Mo+0.5 W<3.0 wt %, 0.05 wt %≦V≦1.0 wt %, and the balance Fe and inevitable impurities; in which the highest hardness obtained by tempering at 450° C. or higher after quenching is HRC 61 or more.
 2. The cold work tool steel according to claim 1, further comprising one or more elements selected from: 0.005 wt %≦Se≦0.10 wt %, 0.005 wt %≦Te≦0.10 wt %, 0.0002 wt %≦Ca≦0.010 wt %, 0.005 wt %≦Pb≦0.10 wt %, and 0.005 wt %≦Bi≦0.10 wt %.
 3. The cold work tool steel according to claim 1, further comprising one or more elements selected from: 0.01 wt %≦Cu≦2.0 wt %, 0.01 wt %≦Ni≦2.0 wt %, 0.20 wt %≦Co≦1.0 wt %, and 0.0003 wt %≦B≦0.010 wt %.
 4. The cold work tool steel according to claim 2, further comprising one or more elements selected from: 0.01 wt %≦Cu≦2.0 wt %, 0.01 wt %≦Ni≦2.0 wt %, 0.20 wt %≦Co≦1.0 wt %, and 0.0003 wt %≦B≦0.010 wt %.
 5. The cold work tool steel according to claim 1, further comprising one or more elements selected from: 0.0010 wt %≦P≦0.030 wt %, 0.0050 wt %≦N≦0.050 wt %, 0.0010 wt %≦Al≦0.10 wt %, and 0.0002 wt %≦O≦0.010 wt %.
 6. The cold work tool steel according to claim 3, further comprising one or more elements selected from: 0.0010 wt %≦P≦0.030 wt %, 0.0050 wt %≦N≦0.050 wt %, 0.0010 wt %≦Al≦0.10 wt %, and 0.0002 wt %≦O≦0.010 wt %.
 7. The cold work tool steel according to claim 4, further comprising one or more elements selected from: 0.0010 wt %≦P≦0.030 wt %, 0.0050 wt %≦N≦0.050 wt %, 0.0010 wt %≦Al≦0.10 wt %, and 0.0002 wt %≦O≦0.010 wt %.
 8. The cold work tool steel according to claim 6, further comprising one or more elements selected from: 0.010 wt %≦Nb≦0.10 wt %, 0.005 wt %≦Ta≦0.10 wt %, 0.005 wt %≦Ti≦0.10 wt %, 0.005 wt %≦Zr≦0.10 wt %, 0.005 wt %≦Mg≦0.10 wt %, and 0.005 wt %≦REM≦0.10 wt %.
 9. The cold work tool steel according to claim 7, further comprising one or more elements selected from: 0.010 wt %≦Nb≦0.10 wt %, 0.005 wt %≦Ta≦0.10 wt %, 0.005 wt %≦Ti≦0.10 wt %, 0.005 wt %≦Zr≦0.10 wt %, 0.005 wt %≦Mg≦0.10 wt %, and 0.005 wt %≦REM≦0.10 wt %.
 10. The cold work tool steel according to claim 1, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 11. The cold work tool steel according to claim 2, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 12. The cold work tool steel according to claim 3, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 13. The cold work tool steel according to claim 5, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 14. The cold work tool steel according to claim 6, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 15. The cold work tool steel according to claim 7, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 16. The cold work tool steel according to claim 9, which has 0.10%≦dA60×400≦1.50%, wherein “dA60×400” is the content of inclusion measured based on a method described in JIS G0555.
 17. The cold work tool steel according to claim 1, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 18. The cold work tool steel according to claim 2, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 19. The cold work tool steel according to claim 3, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 20. The cold work tool steel according to claim 5, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 21. The cold work tool steel according to claim 10, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 22. The cold work tool steel according to claim 11, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 23. The cold work tool steel according to claim 6, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 24. The cold work tool steel according to claim 12, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 25. The cold work tool steel according to claim 13, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 26. The cold work tool steel according to claim 7, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 27. The cold work tool steel according to claim 22, further comprising one or more elements selected from: 0.01 wt %≦Cu≦2.0 wt %, 0.01 wt %≦Ni≦2.0 wt %, 0.20 wt %≦Co≦1.0 wt %, and 0.0003 wt %≦B≦0.010 wt %.
 28. The cold work tool steel according to claim 14, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 29. The cold work tool steel according to claim 15, which can be obtained by quenching at a temperature of 950° C. or higher and 1080° C. or lower with 3.0≦Gq≦8.0, wherein “Gq” is the grain size of the prior austenite after quenching, which is measured based on a method described in JIS G0551.
 30. The cold work tool steel according to claim 28, further comprising one or more elements selected from: 0.010 wt %≦Nb≦0.10 wt %, 0.005 wt %≦Ta≦0.10 wt %, 0.005 wt %≦Ti≦0.10 wt %, 0.005 wt %≦Zr≦0.10 wt %, 0.005 wt %≦Mg≦0.10 wt %, and 0.005 wt %≦REM≦0.10 wt %. 